The present invention relates to the methods of implementation of surface-emission semiconductor lasers, and to the lasers thus implemented.
The method according to the present invention can be advantageously used for implementing several lasers on a single substrate, thus forming a laser array. According to a characteristic, these lasers can be individually controlled.
A semiconductor laser is essentially constituted by an active medium, or "lasing medium", of the crystalline type in which the light is emitted by radiative recombination of the electrons and holes. The active medium is placed in a resonant optical cavity formed by two reflecting mirrors of which at least one is partially transparent to allow a portion of the light emitted within the optical cavity to output from this cavity. Electrons and holes are introduced in the active medium by regions of the n type and of the p type which are disposed in contact with the active medium.
Several variants of semiconductor lasers are known from the prior art and use various active media, various mirrors, various p and n materials, various sizes and various geometries However, the basic principles are the same in every case.
The active medium can be based, for example, on gallium arsenide (GaAs), indium phosphide (InP), or other III-V compounds; or alternatively certain II-VI or IV-VI compounds could also be used as active medium although their technology is less mature.
The main point is that the active medium must have an electron energy forbidden band narrower than the forbidden band of the adjacent n and p materials which provide the electrons and the holes, respectively.
The mirrors may be made of (conductive) metal or of dielectric multilayers or of semiconductors. In some implementations, a mirror can also be used to supply current to the active medium.
The constraints of the approach chosen for implementing a semiconductor laser will have repercussions on cost and yield, on the reproducibility and reliability of the implemented lasers, and on the energy efficiency of the lasers, then on the current consumption, the thermal dissipation and the lifetime of the lasers.
In general, it is desired to fabricate a laser with a highest possible gain for a given current, or a lowest possible threshold current for a given gain. In this respect, the multiple quantum-well (MQW) laser exhibits definite advantages because the threshold current is reduced proportionally to the volume of the active region.
Roughly, one must inject about 10.sup.7 A/cm.sup.3 of current in the active region to trigger laser action. For a layer of 0.1 micron, for example, the threshold current is on the order of 10.sup.3 A/cm.sup.2. In a conventional laser with double heterostructure, the thickness of the epitaxial layers in the active region is of about one to a few hundreds of nanometers. In a MQW laser, the active layers have a thickness of a few nanometers and the density of the threshold current is decreased by about an order of magnitude.
However, to benefit from this advantage, it is necessary to implement a MQW laser with very low losses and consequently with very good mirrors. In addition, the crystalline structure of the active medium must be perfect: any mesh defect condemns the laser to premature failure.
This leads to the use of epitaxial or single-crystal growth methods such as, for example, those of metallo-organic chemical vapor deposition (MOCVD) or alternatively that of molecular-beam epitaxy (MBE) so as to control the thickness of the epitaxial layers to within an atomic layer while achieving a lattice mesh defect level as low as possible. Other growth methods can be envisaged but are not yet fully validated in industrial practice, such as chemical-beam epitaxy (CBE) and molecular-beam epitaxy of metallo-organic compounds (MOMBE).
On either side of the active medium, mirrors are placed so as to create a cavity in which the laser light is amplified. At least one of the two mirrors is semitransparent so that it is possible to extract a portion of the optical power from within the cavity toward outside for use.
For the MQW lasers with surface emission, very good mirrors are required to obtain losses as low as possible within the cavity.
The mirrors may be a metal directly deposited on the layers of the active region, or alternatively a dielectric, therefore insulating. In the present invention, a non-limitative preferred embodiment uses mirrors formed by a stack of various layers of dielectric to form a so-called "Bragg mirror". In a Bragg mirror, each layer of dielectric with a thickness of a few tenths of a micron only, is highly transparent. Stacking alternate layers of different compositions and index of refraction causes the reflection of light in the interfaces between these single-crystal layers obtained by epitaxial growth, due to the difference in refractive index. In the case of epitaxially grown heterostructures with a lattice incoherence between the compounds constituting the successive layers, the latter may be constructed as superlattices where the lattice incoherence is insufficient to cause the creation of defects in the interface of the layers of different compounds, provided the layers are sufficiently thin (a few atomic layers).
The emitted light is constituted by photons produced by the radiative recombination of the electron-hole pairs. The charge carriers--electrons and holes--are supplied to the active region by n and p materials located in the immediate vicinity and in physical contact with the active region. In order to obtain a maximum efficiency, the distance to be traveled before recombination for a hole as for an electron must be shorter than one carrier diffusion length, that is about one to a few microns. This parameter may be determining for the maximum size of the active region.
Also, in order to optimize the efficiency of the laser, it is necessary to minimize the losses due to non-radiative recombinations which take preferably place on the free surfaces of the active region.
Laser amplification by stimulated emission of radiation is obtained through the presence of the photons in the active region, which photons must be confined to the active region to obtain this amplifier action. The photons are confined by the mirrors in the direction of emission (perpendicular to the surface of the laser in the case of the present invention); photons with any other direction of propagation are not confined and consequently are not amplified. In other words, only the emission occuring in the desired direction is stimulated, thanks to the presence of a population of photons in the corresponding quantum state.
Sending back the photons in the active region by mirrors produces a feedback which reinforces the emission of identical photons and consequetly the amplification of light in the active region. This sending back of the photons can be obtained by means of mirrors which consist of several transparent layers with different refractive indices, the feedback being then a so-called "distributed feedback" and the mirrors are referred to as "distributed Bragg reflectors".
But sending back the photons can also be obtained by means of partial reflections between the layers within the active region itself. The laser is then called a "distributed-feedback laser".
To sum up, we have seen that, to implement a solid-state laser, for one thing, it is necessary to provide charge carriers (electrons and holes) to an active region where they will be confined until their radiative recombination and, for another thing, the resulting photons must be confined in the active region a sufficiently long time to obtain the stimulation of emission.
The first problem encountered in the implementation of surface-emission lasers is to confine the photons and the carriers in the active region, whereas the direction of current and that of light emission is the same in most cases.
A first approach known in the prior art from the French patent No. 88 16215, for example, is to have the current come from the sides of the active region so as to be able to optimize in an independent manner the optical and electrical designs as in conventional lasers with emission through the edges.
Other approaches are also known from the prior art, but each of them has disadvantages. The first arrays of laser diodes with surface emission have been implemented at Bell Laboratories, in Holmded, and at Bellcore, in Red Bank. To decrease the volume of the active region and consequently the threshold current, the lasers implemented in great numbers on a single substrate (up to 2 million lasers per square centimeter) are separated and their diameter defined by a deep anisotropic etching (up to 5 microns depth for lasers of 1 to 5 microns in diameter). The main problem of this method is that the lasers are in contact with air on the sides (they have the shape of a vertical cylinder with the axis parallel to the emission). Etching tends to deteriorate the lattice structure of the etched portion and leaves lattice defects on the edges of the active region.
Moreover, the non-radiative recombination is stronger on the free sides of the active region. Finally, for the integration into an array, possibly with individual control, it is desirable to fill the voids between the lasers, up to planarization. This could increase the cooling of the lasers, and also serve as a support for the control electrical traces; in addition, non-radiative recombinations should be reduced proportionally to the reduction of the free surface area.
In the prior art, two methods are known to fill the voids between the lasers. One method consists in a planarization carried out on the plate comprising the array of lasers by depositing a material by epitaxial growth methods, for example. Such a method turns out to be difficult to implement, for the depth to be filled is of about 5 microns, whereas the width of the voids between the lasers is often of about 1 to 2 microns only.
Another method known from the prior art consists in implementing the laser by the epitaxy methods mentioned above and then to create sources of holes and electrons on either side of the active region through diffusion of impurities ("dopants") p and n, respectively, in the material surrounding the active region.
This method is described in the French application for patent having the French number 88 16215 filed by the present applicant.
In accordance with this method, the p and n impurities are diffused, each respectively in a half-shell surrounding half of each laser diode. This method has the disadvantage of requiring several (at least two but possibly more) steps of implantation and diffusion of impurities in accordance with a precise and alternate geometry.